std::vector::reserve
From cppreference.com
                    
                                        
                    
                    
                                                            
                    |   void reserve( size_type size );  | 
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Sets the capacity of the container to at least size. New storage is allocated if necessary.
Contents | 
[edit] Parameters
| size | - | new capacity of the container | 
[edit] Return value
(none)
[edit] Complexity
linear in the size of the container
[edit] See also
|    returns the number of elements that can be held in currently allocated storage   (public member function)  | |